1. Field of the Invention
The present invention relates to a chip resistor in which a resistor body film is provided on a chip type insulating substrate. More particularly, the present invention relates to a highly reliable chip resistor in which an electrode coming into contact with a resistor body is made of material, which is seldom diffused into the resistor body, the heat-resistance with respect to solder of which is high, so that the electrode can be seldom eroded by solder and the reliability can be enhanced.
2. Description of the Related Art
As related arts of this invention, two types of chip resistors have been known such taht one is a thick film resistor of which electrode and resistor are manufactured by means of printing or baking, and the other is a thin film resistor which electrode and resistor are manufactured by means of spattering method. Although the thickness of the film is different from each other between the thick film resistor and the thin film resistor, the structure of the thick film resistor and that of the thin film resistor are almost the same. For example, the structure is shown in FIG. 5. In FIG. 5, at both end sections of the insulating substrate 1 made of alumina which are opposed to each other, there are provided a pair of electrodes 2, 3 which include upper surface electrodes 21, 31, reverse face electrodes 22, 32 and side electrodes 23, 33 connecting these electrodes. Further, there is provided a resistor body 4 on the insulating substrate 1 in such a manner that the resistor body 4 is connected with both the electrodes. On the surface side of the resistor body, there are provided one to three layers of protective films 5 (51 to 53). In order to easily mount the pair of electrodes 2, 3 on a circuit substrate, Ni-plating layers 25a, 35a and solder-plating layers 25b, 35b are provided on the surfaces of the pair of electrodes 2, 3.
The thick film resistor is made in such a manner that paste-like material made of glass or resin is coated by means of printing and baked at 600 to 900xc2x0 C. in the case of glass or cured at 200 to 300xc2x0 C. in the case of resin. Concerning the electrode material, Ag paste (silver paste) is used in which Pd is added to Ag. Concerning the resistor body material, paste is used in which Ag or Pd is mixed with glass, resin or ruthenium oxide so as to obtain a necessary resistance value. Concerning this thick film resistor, the equipment cost of the manufacturing apparatus is low, and further the thick film resistor can be manufactured in a short period of time in the manufacturing process. Therefore, the manufacturing cost of the thick film resistor is much lower than that of the thin film resistor which must be manufactured by means of spattering. Accordingly, the thick film resistor is conveniently used.
As described above, the thick film resistor can be easily manufactured in the manufacturing process, however, materials of the electrode and resistor body are made of paste in which glass paste or resin paste is mixed, and this mixed paste is coated, and baked or cured. Therefore, for example, when electrode material made of silver paste comes into contact with the resistor body, Ag in the electrode diffuses into the resistor body, so that the characteristic of the resistor body such as a resistance value and a temperature coefficient fluctuates. In order to solve the above problems, it is considered that the upper surface material is not made of Ag paste but Au paste.
On the other hand, even in the case of a chip resistor in which Au paste is used as the electrode material, the following problems are caused. When soldering is conducted on the chip resistor in the process of mounting, the resistance value is suddenly increased or the resistor is put into an open state.
The present invention has been accomplished in view of the above circumstances. It is an object of the present invention to provide a chip resistor having the electrode material being not diffused into the resistor body in the manufacturing processso that the characteristic of the resistor body is not changed, and further, there is no possibility of the chip resistor being not put into an open state in the case of mounting.
The inventors of this present invention have found it out through their hard investigations that the chip resistors, such as showing sudden increase of the resistance value or showing an open state after the completion of soldering in the process of mounting, have their own specific reason. That is to say, said reason is such that upper surface electrode made of material of gold, which is used as material for preventing metal diffusion into the resistor body, is melted into solder on the lower side of an interface between the solder plating layer and the protective film (shown by xe2x80x9cAxe2x80x9d indicated in FIG. 5) by which the upper surface electrode disconnected. In order to prevent the electrode material from being melting into solder, this invention arrives at a conclusion that it is effective to cover the upper surface electrode with electrode material of Ag rather than that of Au from the fact that the melting-resistance of Ag with respect to solder at high temperature is stronger than that of Au.
The present invention provides a chip resistor comprising: an insulating substrate; a pair of upper surface electrodes provided at both end sections of the substrate opposed to each other; a resistor body arranged on the substrate so that both end sections of the resistor body can be electrically connected with the pair of upper surface electrodes; a pair of upper surface auxiliary electrodes made of material, the heat-resistance with respect to solder of which is higher than that of the upper surface electrodes, which are respectively arranged on the pair of upper surface electrodes so that the exposed sections of the upper surface electrodes can be completely covered with the pair of upper surface auxiliary electrodes, the pair of upper surface auxiliary electrodes not directly coming into contact with the resistor body; and a protective film provided on the surface of the resistor body. In this case, the melting-resistance of material with respect to solder at high temperature is defined as a property in which the material is not melted into solder even when the material comes into contact with solder and the temperature of the material is raised.
In the above structure, the upper surface electrode is made of material of Au which is seldom diffused into the resistor body, and the upper surface auxiliary electrode, the heat-resistance with respect to solder of which is high, is provided on the surface of the upper surface electrode. Due to the above structure, there is no possibility that melted solder reaches the upper surface electrode in the process of soldering. Therefore, the upper surface electrode is not melted into solder. On the other hand, since the upper auxiliary electrode is not contacted with the resistor body, when it is baked or cured in the manufacturing process, the material of the upper surface auxiliary electrode is not diffused into the resistor body.
The protective film includes: a first protective film covering a surface of the resistor body in the case of laser beam trimming; a second protective film covering the inside of a recessed groove formed by laser beam trimming after the completion of laser beam trimming, the second protective film completely covering the exposed section of the resistor body; and a third protective film, which is provided on the second protective film, for making the surface flat, wherein the upper surface auxiliary electrodes are arranged overlapping the second protective film and the upper surface electrodes so that the upper surface auxiliary electrodes can not directly come into contact with the resistor body.
Specifically, when the upper surface electrode is made of material of gold and the upper surface auxiliary electrode is made of material of silver, no electrode material diffuses into the resistor body, and it becomes possible to prevent the electrode material from melting into solder in the process of soldering. In this case, the material of gold is defined as material, the primary component of which is Au, capable of containing other elements, and the material of silver is defined as material, the primary component of which is Ag, capable of containing other elements.